HN-66000 is a proprietary pharmaceutical formulation of mutant diptheria toxin (CRM 107) and human transferrin(Tf) that is being developed as a potential treat ment for malignant brain tumors. The results for a Phase I trial with HN-66000, sponsored by the NIH, suggest that HN-66000 produces tumor responses in patients with malignant brain tumors refractory to conventional therapy without severe neurologic or systemic toxicity. These encouraging results warrant an expanded evaluation of HN-66000 at other centers in a Phase II trial.